TSI Data Plots (Updated Daily)

TIM Data Product Description

The Total Irradiance Monitor (TIM) measures the total solar irradiance (TSI), monitoring the incident radiant energy powering the Earth’s climate system. The TIM uses an ambient temperature active cavity radiometer with an at-launch estimated absolute accuracy of ~400 parts per million (ppm, 1 ppm=0.0001%) (1-sigma) and an expected long-term relative accuracy (stability) of 10 ppm per year. These uncertainties are included in the released Level 3 data files. The available Level 3 TSI data products produced by the TSIS program consist of daily and 6-hourly average irradiances reported at a mean solar distance of 1 astronomical unit (AU) and zero relative line-of-sight velocity with respect to the Sun. At-Earth values are also included for climate researchers to use as inputs to models.

Two TSI data files are produced daily. One contains the daily means and the other contains four 6-hourly means per day. Due to the small size of the daily data and to maximize ease of use to end-users, each delivered TSI product contains science results for the entire mission. Lower-level data products (e.g. Level 2) have limited scientific value due to frequent gaps caused by the Earth occulting the Sun in the TSIS-1’s low-Earth orbit, and are therefore not delivered to the GES DISC. Short duration time periods of these data can be made available upon request.

Data Quality Description

On-orbit instrument characterization is an on-going effort, as the TIM team regularly tracks instrument degradation and calibrates the instrument servo system on-orbit, periodically updating the data processing system with new calibration values. Only minor corrections are anticipated at this phase in the TSIS/TIM mission.

TIM Data Production Overview

The TIM measures the absolute intensity of solar radiation integrated over the entire solar irradiance spectrum. To construct this product, high time cadence measurements (approximately every 50 seconds during sunlit portions of the TSIS spacecraft orbit) from the instrument are combined to produce representative daily and 6-hourly values of the TSI. Four TIM radiometer channels track on-orbit degradation in the primary channel and additionally provide a limited degree of redundancy. Shuttered operation of the instrument corrects for thermal background, and state-of-the-art phase sensitive detection algorithms applied to the data at the shutter fundamental reduce noise and sensitivity to drifts. The TIM shutter period is 100 seconds; four such cycles are weighted to give a single irradiance measurement at a cadence of 50 seconds. These so-called “Level 2” data are averaged over an entire day as well as over each 6-hour interval to formulate the two primary TSIS TSI data products that are released to the GES DISC. The TSI value and the time stamp reported are means of the TSI measurements acquired during the interval.

TSI Research and Applications

Measurements of TSI are known to be linked to Earth climate and temperature. Proxies of the TSI based on sunspot observations, tree ring records, ice cores, and cosmogenic isotopes have given estimates of the solar influence on the Earth that extend back thousands of years, and correlate with major climatic events on the Earth. These estimates extrapolate many recent detailed observations to long-term observations of fewer (or even one) measurement. For example, accurate TSI measurements from the last 36 years are correlated with solar measurements of sunspots and faculae; these correlations can then be used to extrapolate the TSI to time periods prior to accurate space-borne TSI measurements, since solar records extend back approximately 100 years for faculae and over 400 years for sunspots. Over this extended time range, the extrapolated TSI record can be compared with longer term records, such as tree rings or ice cores, and correlation with these allows extension of the estimated TSI to more distant historical times, albeit with decreasing certainty. This extrapolation is important for understanding the relationship between TSI and the Earth’s climate; yet the extrapolation begins with the comparison of solar surface features to accurate TSI measurements, a record which is currently only 36 years long. Good accuracy and stability in this recent record are thus of high importance and drive the measurement objectives of the TIM.

TSI Measurement History

Attempts to measure the TSI began in earnest in the 1830’s, with independent measurements by Claude Pouillet and John Herschel, yet were nearly a factor of two low because of atmospheric absorption. Even balloon-borne measurements in the 1900’s lacked the instrumental accuracy to detect the ~0.1% changes in the TSI. It was not until long-duration measurements from space were available that changes in TSI were accurately measured and the misconception of a “solar constant” changed. TSI monitoring using electrical substitution radiometers (ESRs) from the vantage point of space began with the launch of the Nimbus 7 satellite in November 1978. This was soon followed by an Active Cavity Radiometer Irradiance Monitor (ACRIM) instrument on the Solar Maximum Mission and by the Earth Radiation Budget Experiment (ERBE). More recently, second and third ACRIM instruments have been launched, in addition to the launch of the VIRGO on the NASA/ESA Solar and Heliospheric Observatory (SoHO). The various data sets are in basic agreement and show conclusively that variations of TSI track the passage of sunspots across the solar disk with an amplitude of about 0.2%, and that long-term solar-cycle variations are only on the order of 0.1%. The TSIS TSI data set continues these important observations to extend this uninterrupted multi-decadal solar climate data record.

Of the mentioned TSI instruments, VIRGO, SORCE, and TCTE are the only ones from prior to the TSIS launch that continue to make observations.

Instrument Description

Scientific Contact

References

T. Dudok de Wit, G. Kopp, C. Fröhlich, and M. Schöll, “Methodology to create a new Total Solar Irradiance record: Making a composite out of multiple data records,” Geophysical Research Letters, 2017, doi:10.1002/2016GL071866.